Adding the GMF source files to your source path

The easiest way to do this is to go to the software update manager thing, and download the GMF SDK -- similar to the method used to install EMF/GMF in the first place.

Cannot insert new nodes into an EMF editor

If you cannot right click and add children to a wizard-generated model in your EMF-generated plugin workspace, ensure that the properties "children", "create child" and "notify" in the .genmodel are set to true. (This may be because "Containment" isn't set to true in the underlying .ecore model: See GMF Troubleshooting.)

DanglingHREFException

If when creating a new model, trying to add a child to a node, you get a DanglingHREFException, make sure that in the .ecore model, the particular EReference has the "containment" property set to true. (ref)

This has the effect of setting a reference from merely an association into an aggregation.

Top level node 'Containment Feature' must be available in the diagram 'Domain Element' or its super-class

I think this means that you are trying to make GMF create an editor for a model element that can't actually be displayed in the current editor. e.g. You are trying to make a model, based in Map, which displays both Topic and TopicConcept, but you cannot currently create TopicConcept in the Map object. Or something... I haven't quite worked this one out yet

The object 'SomethingImpl' (text: content) is not contained in a resource

This error occurs in the Eclipse workspace when you try to save a model which has an invalid structure -- commonly when it refered to an old .ecore metamodel. The solution to this is to edit the model with a text editor and remove the invalid model elements.